Diode-lasers are preferred as pump-light sources for solid-state lasers, inter alia, because they can be designed to emit at essentially any wavelength within a wavelength range determined by particular semiconductor materials used for their manufacture. The wavelength (pump-wavelength) can be selected to match a preferred absorption band of a solid-state gain-medium being pumped. This provides that essentially the entire output of the diode-laser is effective in pumping the gain-medium. This, together with the well-known efficiency of the diode-laser, provides a corresponding high efficiency for the DPSS-laser as a whole.
One disadvantage of such a highly selective pumping arrangement, however, is that if the emitting-wavelength of the diode-laser changes, the efficiency of the DPSS-laser can change dramatically. The output-wavelength of a diode-laser, at any given temperature, is known to change, over time, as the diode-laser is used, i.e., with operational "aging". Accordingly, if the performance of a DPSS-laser is to be maintained at an optimum level, some means must be provided for maintaining the emitting-wavelength of the diode-laser at the pump-wavelength.